1. Field of the Invention
The present invention generally relates to solar cells and solar cell panels and in particular to solar cell interconnects. More specifically, the invention relates to an improved in-plane solar cell interconnect that supports a diode on one side of the solar cell.
2. Description of Related Art
With the ready availability of solar energy in outer space for a spacecraft such as a satellite, the conversion of solar energy into electrical energy with photovoltaic cells is an obvious choice for producing power. Higher efficiency in power conversion of sunlight to electricity equates to either lighter weight spacecraft or higher payload capacity, both of which have monetary benefit. One method for increasing efficiency is to manufacture solar cells with multiple junctions, or layers having different energy band gaps which have been stacked so that each cell or layer can absorb a different part of the wide energy distribution in the sunlight. Because of the high voltage of these cells compared to silicon and their susceptibility to reverse bias breakdown, there is a requirement to protect each cell with a bypass diode. Attachment of the diode to each cell is in addition to attaching interconnects for the purpose of increasing voltage in a solar cell circuit by series connection. A number of the stacked arrangements or cells is provided in the form of an array on a substrate or multiple substrates.
Interconnects are used in an array of solar cells to electrically connect the cells to one another in series, in parallel or both. They must be designed robust to survive the rigors of the space environment. The individual solar cells and their substrate can be subject to significant mechanical vibration during a launch and thermal cycling during the course of the spacecraft""s mission in space. The thermal cycling, in turn, leads to thermal expansion and contraction of the various materials. This can cause stress on the interconnects if there is a coefficient of expansion (CTE) mismatch between the cells and the substrate materials. With greater stress in terms of frequency and magnitude, there can be a shorter life expectancy of the interconnect. With a shorter life expectancy of the interconnect, a solar panel in which the interconnect is utilized will have a shorter life expectancy. Ultimately, the spacecraft on which the solar panel is used will have a shorter life and result in greater costs to replace it.
Various interconnect designs have been attempted in the past. Examples of various interconnects can be found in U.S. Pat. Nos. 5,006,179; 4,193,820; and 3,819,417. Some of the disadvantages of these designs include the need for two types of connections made from two different materials, which tends to complicate manufacturing. Another disadvantage is that the interconnects take up or shade active areas of the tops of the cells. Further, some prior designs provide insufficient flexibility between the cells or prevent the cells in an array from being close to one another, thus increasing the area and mass requirements of the array.
The foregoing designs do not address the issue of an interconnect in the context of a solar cell assembly having a bypass diode. One prior art design, however, that does so is U.S. Pat. No. 5,616,185 wherein a flexible interconnect tab is used to connect a p-contact on one solar cell assembly to an n-contact on an adjacent solar cell assembly. Each solar cell assembly includes a bypass diode on the back (or non-illuminated) side of the solar cell. The diode is connected in opposite polarity with its solar cell so as to provide junction breakdown protection for the solar cell by limiting the reverse voltage that can occur across the cell. Thus, a conductive interconnect that is separate from the interconnect tab connects the anode contact of the diode to the p-contact of the next assembly. However, a drawback to this design is the difficulty in removing and replacing an individual solar cell from an array without damaging adjoining cells and structures.
FIG. 1 depicts another interconnect design that has been used with solar cell assemblies containing a diode on the back side. A first solar cell assembly 10 includes a cover glass 12 disposed over an ohmic bar with adhesive 13. The ohmic bar is on the top surface of a solar cell 14. A wraparound diode tab 17 is fixed to a front side of the assembly 10 and wraps around to the back side of the assembly 10 to hold a diode 16 in place on the back side. Between the diode tab 17 and the solar cell 14 is an insulation 18. An adhesive layer 15 adheres the diode 16 to the solar cell 14. In FIG. 1, an interconnect 19 electrically connects the first solar cell assembly 10 to a second solar cell assembly 11. Typically, three interconnects 19 are used between the adjacent solar cell assemblies 10 and 11, such as when each assembly is about 7 cmxc3x973.5 cm.
For the design in FIG. 1, the need for multiple parts to accomplish connection between the assemblies 11, 12, as well as electrically connect the diode 16, results in multiple processing and handling steps. In turn, there is not only increased manufacturing time and expense, but also more possibilities of solar cell attrition clue to breakage. Additionally, since the diode tab 17 and multiple interconnects 19 are typically welded at the ohmic bar, each weld pulse can degrade the electrical performance of the solar cell assembly 10 due to shunting. A small shunt reduces the electrical output of the solar cell 14 by leaking current across the junction or junctions of the cell 14, thus reducing its conversion efficiency. A severe shunt results in rejecting the expensive cell 14 completely.
Furthermore, the diode tab 17 places a strain on the ohmic weld joint and cover glass 12. Because of the thermal and mechanical stresses imparted on the bonded solar cell 14 in long life missions, the interconnect 19 is typically made with a strong material such as Kovar, Invar, and molybdenum. On the other hand, the strong materials make the interconnect 19 stiffer. When the diode tab 17 is bent around in a wraparound configuration, the tab 17 has a tendency to both delaminate or even shatter a thin cover glass 12, as well as to de-weld from the ohmic bar on the front surface of the solar cell 14. This condition can render the diode 16 useless for protecting the cell 14 from reverse current and also can reduce the ability of the cover glass 12 to protect the cell 14 from radiation damage. These two conditions can then reduce the power of an entire solar cell circuit connected in series with the damaged cell 14.
As can be seen, there is a need for interconnects that achieve the following functions or have the: following characteristics electrical connection between adjacent solar cells, as well as back to front connection for a diode mounted on the backside of a single cell; improved resistance to thermal cycling; flexure in an in-plane configuration; connection to the cell with a minimum number of processing steps and cell interconnection that minimizes the number of welds so that the potential for cell damage is minimized. An inplane solar cell interconnect is also needed to reduce the potential amount of shading to active areas of the cell and to facilitate cleaning of bonding adhesive off the top surface of at cover glass.
The present invention is directed to an improved interconnect for connecting first and second solar cells. The interconnect comprises a base portion having a first distal section and a first intermediate section, with the first distal and intermediate sections being integral with the base portion and capable of fixture to the first solar cell. An extension tab is integral to the first distal section, with the extension tab being capable of fixture to a front side of the second solar cell. A diode tab is integral to the first intermediate section, with the diode tab being capable of fixture to a diode side of the second solar cell, such diode side being on a side opposite to the front side. The present invention is applicable both to solar cells used for terrestrial applications as well as non-terrestrial applications such as on satellites and airplanes and high altitude platforms and vehicles.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description, and claims.